1. Field of the Invention
This invention relates to UV and e-beam curable, transparent and abrasion resistant coatings which adhere tenaciously to metals, especially aluminum, brass, silver and nickel surfaces and which protect said surfaces from discoloration by corrosion and/or by exposure to boiling water. Yet these coatings can easily be stripped from said metal surfaces by exposure to 15% ammonium hydroxide solutions.
2. Background of the Art
One of the most common reasons for discoloration of metals is their tendency to corrode under the influence of the environment. A very common example of corrosion is the rusting of iron in any humid atmosphere and the subsequent destruction of its surface. Ordinarily, most common metals such as iron, steel, aluminum and their various alloys have been protected from corrosion by coating those substrates with paints, lacquers and varnishes, or in other cases by anodizing their surfaces or electroplating and deposition of protective metallic coatings. Metal alloys are also used which are more resistant to corrosion than the metals themselves (e.g., stainless steel), but the improvement in corrosion resistance is often at the sacrifice of other properties.
Although other metals such as copper, silver, nickel and their alloys are much more resistant to corrosion than iron, it is well known that these metals too are subject to some corrosion and subsequent tarnishing, especially at relatively high temperatures. For example copper and its alloy, brass, are capable of resisting attack by pure steam, but if much carbon dioxide, oxygen, or ammonia is present, the aqueous condensate becomes corrosive. Condensates containing 5 ppm of oxygen and 15 ppm of carbon dioxide have been shown to have a corrosion rate of 0.18-0.35 mm/year in contact with copper alloys. Another environment which causes brass and copper to corrode and discolor is saltwater. For example, in quiet seawater, copper and brass corrode at a rate nearing 50 micrometer/year. In the case of silver, copper, and brass, one of the most common causes of discoloration is the reaction of the surface with sulfur-containing chemicals, especially SO.sub.2 and H.sub.2 S. In the case of silver, hydrogen sulfide in the atmosphere, which is one by-product of the burning of sulfur-containing fossil fuels, causes most of the tarnish. In the case of brass and copper, moisture is needed in combination with H.sub.2 S and SO.sub.2 for corrosion to take place.
In many industrial, household, and decorative products where brass, silver, aluminum and nickel metals are used, the need is for a protective coating which not only protects the surface from discoloration, but which in certain cases, such as bathroom or kitchen fixtures, resists extended exposure to hot water and household chemicals while retaining its aesthetic appearance, abrasion resistance, transparency, lubricity and the like. Clear lacquers are sometimes used, as are silicone coatings, epoxy coatings, or various combinations of multilayers thereof. For example Incralac.RTM., a clear lacquer for brass developed by the International Copper Research Institute preserves the bright copper or brass color for decorative reasons. However, inherent in these organic coatings are several properties that remain in need of improvement, including low abrasion resistance and poor adhesion to the metal, especially after exposure to a moist environment. For example, no lacquer based coating on the market today can withstand abrasion by a fine steel wool pad, such as Steel Wool #0000, without visible surface damage.
The problem of poor adhesion of protective coatings to metals in general has been addressed in many ways. For example, primers which couple top coats to the substrates are known. Patent publications abound with primer technology, however, primers are not universal and must be matched according to both the nature of the coating and the nature of the substrate.
Adhesion additives, such as silane and titanate coupling agents, have also been used extensively. These additives function by co-reacting in situ with the coating and then reacting with the substrate when the coating is laid on the substrate to give the bonding effect (e.g., U.S. Pat. No. 4,396,650).
Surface roughening or texturing followed by coating with various polymers and the subsequent surmounting of such polymers by the protective overcoat is another known method for promoting adhesion to metal substrates.
Still, in the case of such metals as brass, nickel, and silver the above priming methods have proved inadequate or insufficient to overcome the problem of their poor adhesion to protective organic coatings.
Abrasion resistant coating with good adhesion to metals were described in U.S. Pat. No. 4,243,722 (Haluska) and U.S. Pat. No. 4,742,111 (Chi). These compositions are aqueous dispersions of colloidal silica in lower aliphatic alcohol-water solution of a partial condensate of a mixture of silanols of the formula RSi (OH).sub.3 wherein R is an alkyl radical or a substituted alkyl radical containing, among other groups, mercaptyl groups acting both as anti-tarnishing agents and as adhesion promoters. A phenolic resin was added in the compositions disclosed by Chi. The compositions described in the Haluska patent and Chi patent are very similar to abrasion resistant compositions described for example in U.S. Pat. Nos. 3,976,497; 3,986,997; 3,708,285; 4,368,236; and 4,680,232.
Although the coatings disclosed by Haluska and Chi provide adequate adhesion to gold and silver when hydrolyzed mercaptosilanes are included, they suffer from several major drawbacks which render them impractical or unsuitable for certain industrial applications. These drawbacks are inherent in all compositions which rely on hydrolyzed silanes or functionalized silanes in the formulation of protective coatings. One drawback is that the shelf-life of the partially hydrolyzed silanes or functionalized silanes is often limited due to the progressive gelling of the condensation polymer, as is documented in U.S. Pat. No. 3,986,997 for example. Normally this progressive gelling may be slowed down somewhat, but not eliminated, when the solution is stored at or below 4.degree. C. as is usually recommended by the manufacturer. A second drawback of these materials is that they are relatively slow in curing, requiring normally one hour of precure at room temperature, followed by from two to fifteen hours of bake time. A third drawback is that the cure temperatures required are relatively high. For example, in example 2 of the Haluska patent the bake temperatures varied from 80.degree. C. to 125.degree. C., and the bake times were 15 hours. Last, but not least, is that abrasion resistant coatings derived from aqueous dispersions of colloidal silica in partially hydrolyzed silanes are incapable of withstanding immersion in boiling water or extended exposure to steam without shrinkage, cracking and delamination, probably due to additional crosslinking reaction due to hydrolysis of unreacted alkoxy groups in the silane molecules.
In many decorative and reflective products (including plumbing fixtures) which involve vapor coated or sputter coated metals on synthetic thermoplastic films, the materials disclosed by Haluska are entirely impractical for protecting the metal films because of the high bake temperatures and the long bake times required. What is desired for these decorative products are primerless, well adhering protective films that cure at high speeds and at temperatures much below the thermal distortion temperatures of the thermoplastic substrate. Ideally, radiation curable compositions whereby UV or e-beam irradiation is used to harden the protective coating are desired. Additionally, these products would use important advantage of radiation curable protective coatings, their relative insensitivity to moisture at high temperature resulting from their cure mechanism being through free radical induced polymerization.
In U.S. Pat. No. 4,348,462 (Chung) photocurable compositions based on mixtures of (a) glycidoxy and acryloxy functional silanes, (b) non-silyl acrylates and (c) colloidal silica, have been disclosed. Although it has been stated in the Chung patent that these compositions provide abrasion resistant coatings for metals, without specifying which ones, it has been our experience that these compositions do not adhere to silver and brass without a suitable primer layer and in particular, as stated in U.S. Pat. No. 4,243,722 (Haluska), a mercapto-functional silane is needed for adhesion to silver (and gold). Additionally, it has been our experience that compositions containing photocurable (i.e. acryloxy and glycidoxy functionalized) silanes are not resistant to steam and extended exposure to moisture (for example immersion in boiling water for more than ten minutes), in part because of additional hydrolysis and subsequent condensation of unreacted alkoxy groups.
Non-silyl photocurable compositions based on multifunctional acrylate or methacrylate monomers such as the ones described in U.S. Pat. Nos. 3,968,305 or 4,262,072, are widely accepted alternatives to silane-based hardcoats for protecting thermoplastic substrates. Despite their desirable properties in so far as ease of crosslinking, transparency, chemical inertness and abrasion resistance, the adhesion of top coats derived from polyfunctional acrylates or methacrylates to the metals silver, brass, nickel and aluminum is very poor. No radiation curable hardcoat is known to have been shown so far to exhibit long-term adhesion to silver, brass, or nickel under the corrosive environments discussed above. One reason for the lack of adhesion of radiation curable monomers, such as polyfunctional acrylates, to unprimed rigid surfaces, including metal surfaces, is believed to be the excessive shrinkage of the coating due to the crosslinking reaction. Normally, 10 to 20% shrinkage (measured by dilatometry) is expected on polymerization. Reference: "Principles of Polymer Systems," F. Rodriquez, p. 97, McGraw-Hill (1976). This shrinkage leads to film cracking and/or interfacial stresses which cause either immediate or progressive adhesion failure. The coatings of the present invention exhibit no apparent shrinkage or cracking upon cure and this is believed to be the result of the high inorganic oxide content.